1. Field of the Invention
The present invention relates to a fluid-filled vibration-damping device of tubular design adapted for use as an automotive suspension bushing, for example.
2. Description of the Related Art
Vibration-damping devices has been used to be interposed between components that make up a vibration transmission system for vibration damping linkage between the components. One known vibration-damping device of tubular design includes an inner shaft member, an outer cylindrical member disposed about the inner shaft member, and a main rubber elastic body linking the inner shaft member and the outer cylindrical member mutually. Additionally, fluid-filled cylindrical vibration-damping devices that utilize damping action on the basis of flow of a fluid filling the interior, have widely known and employed as an automotive suspension bushing, for example.
One requirement for a fluid-filled cylindrical vibration-damping device of this kind is that the device exhibit effective vibration-damping action against vibration of each of a number of different frequency ranges. However, the conventional devices encountered the problem that where a low-frequency orifice passage tuned to vibration in a low-frequency range is provided with a view to ensuring satisfactory vibration-damping action on the basis of flow action of fluid induced to flow through the low-frequency orifice passage, vibration-damping ability against vibration in a higher frequency range than that of the low-frequency orifice passage may be diminished appreciably due to antiresonance on the part of the low-frequency orifice passage.
To address this problem, there have been proposed various structures provided with a short circuit channel disposed parallel to the orifice passage, and with an elastic element disposed interrupting the short circuit channel so that the elastic element constitutes a relief mechanism adapted to switch the short circuit channel between an open state and a cutoff state. Examples of such designs include the sealing lip (12, 12′) appearing in FIG. 3 of U.S. Pat. No. 6,168,144B1; and the rubber lip piece 9 appearing in FIG. 3 of Japanese Unexamined Patent Publication No. JP-A-01-255736.
However, these relief mechanisms of conventional structure have all been designed such that the elastic element will experience elastic deformation on the basis of a relative pressure differential between two liquid chambers, thereby placing the short circuit channel in the open state. For this reason, in actual practice, operation is inconsistent, making it difficult to consistently achieve the intended vibration-damping performance. Particularly where the elastic element is made of an elastic rubber body, due to the difficulty of ensuring good dimensional accuracy during the molding process as a result of factors such as molding shrinkage and to variation in contact force against the wall of the short circuit channel arising in the diameter-reduction process of the outer sleeve that has been vulcanization-bonded to the outer peripheral face of the main rubber elastic body, it has proven difficult to achieve a relief mechanism that will operate in a reliable manner when acted on by an intended level of pressure.
While it would be conceivable to make it easier beforehand for the elastic element and the wall of the short circuit channel to separate so that the short circuit channel will reliably assume the open state when acted on by an intended level of pressure. This arrangement poses a problem that the elastic element will experience elastic deformation even at very low pressure levels, causing the short circuit channel to open. This makes it difficult to ensure sufficient flow of fluid through the low-frequency orifice passage so that vibration-damping ability against vibration of the low-frequency range to which the low-frequency orifice passage has been tuned is diminished.